Extravasation of soluble factors such as albumin or immunoglobulins is a direct correlate of BBB dysfunction as visualised by Gd-enhanced MR imaging [23, 30]. partially prevented sVCAM-1-induced changes of tight junction arrangement. Importantly, natalizumab, a neutralising recombinant monoclonal antibody against integrin -4 approved Nelarabine (Arranon) for the treatment of patients with relapsingCremitting MS, partially antagonised the barrier-disturbing effect of sVCAM-1. In summary, we newly characterised sVCAM-1 as a compromising factor of brain endothelial barrier function that may be partially blocked by the MS therapeutic natalizumab. Keywords: Multiple sclerosis, BloodCbrain barrier, Endothelial cell, Integrin alpha4, Vascular cell adhesion molecule-1, Natalizumab Introduction Cell-bound vascular cell adhesion molecule-1 (VCAM-1, CD106) allows human brain microvascular endothelium to control immune cell trafficking across the bloodCbrain barrier (BBB). It is upregulated in inflammatory-active brain lesions of patients with multiple sclerosis (MS), a chronic degenerative autoimmune disease of the CNS [1, 16, 45]. Endothelial VCAM-1 serves as a binding partner for integrin -4/-1 (very late antigen-4, VLA-4) on Nelarabine (Arranon) peripheral blood mononuclear cells (PBMC), as it does for -4/-7 heterodimers to a lesser extent [5, 17]. This molecular interaction enables PBMC to firmly adhere to the vessel wall after rapid activation of integrin -4-mediated intracellular signalling cascades, allowing subsequent immune cell extravasation [39]. A soluble form of VCAM-1 (sVCAM-1) is shedded from the surface of brain endothelial cells upon inflammatory activation [24]. In vitro, sVCAM-1 blocked leukocyte adhesion to activated Nelarabine (Arranon) human brain endothelium. Soluble VCAM-1 was therefore considered as an inflammation-limiting factor at the inflamed BBB [25]. Clinical studies in MS patients treated with recombinant interferon- (IFN-), which moderately reduces relapse rate, disability progression and MRI disease activity [9], seemed to support this hypothesis: IFN- therapy increased sVCAM-1 serum levels, which correlated with a reduction of gadolinium (Gd)-enhancing MRI brain lesions, indicating less inflammatory disease activity at the BBB [12, 20, 38]. Together, these in vitro and clinical studies suggested a local anti-inflammatory effect of sVCAM-1 at the human BBB due to an inhibition of immune cell extravasation [14]. In addition to their regulation of immune cell trafficking, brain endothelial cells strictly govern Nelarabine (Arranon) the exchange of solute and soluble factors across the BBB. Endothelial molecular control mechanisms include active transendothelial transport systems and tight junctions. The latter are highly dynamic trans-membrane protein complexes, tightly sealing the interendothelial clefts [46]. Extravasation of soluble factors such as albumin WNT-12 or immunoglobulins is a direct correlate of BBB dysfunction as visualised by Gd-enhanced MR imaging [23, 30]. Interestingly, sVCAM-1 serum levels in MS patients not receiving IFN- treatment were shown to positively correlate with the presence of Gd-enhancing lesions on brain MRI scans and with Nelarabine (Arranon) clinical disease activity in a majority of studies [15, 18, 19, 22, 36, 37]. This seems to contradict the IFN- studies cited above where an inverse correlation between sVCAM-1 serum levels and MRI disease activity was observed. The mechanisms underlying these divergent findings are unknown. Furthermore, it currently remains unknown whether sVCAM-1 exerts any direct effect on brain endothelial barrier function. So far, it is unknown whether undiseased human brain endothelial cells or those in MS CNS lesions express the established binding partners of sVCAM-1, i.e. integrin -4 heterodimers. An expression of integrin -4 by human brain endothelium in situ was previously described in activated glioma endothelial cells [33]. Furthermore, expression of integrin -4/-1 and of -4/-7 was reported in human umbilical vein endothelial cells (HUVEC) and in adult human synovial membrane endothelium from patients with rheumatoid arthritis [6, 29]. VLA-4 expression was furthermore documented in adult human dermal microvascular endothelial cells [26]. Endothelial integrin -4 expression was lower than integrin -1 expression in these endothelial cell types, but could be upregulated by pro-inflammatory stimulation with TNF- [6, 32]. HUVEC binding to recombinant VCAM-1 and to the extracellular matrix protein fibronectin was found.